Axial piston pump-motor system



0 United States tet [11] 3,535,

[72] Inventor Arthur EAmierson 3,116,595 1/1964 Kent at al. 1l03/162XElmhursl, Illi i 3,237,570 1/1966 Schott 103/162 [21] AppLNo. 716,7353,240,159 3/1966 Andrews et al. 103/162 [22] Filed March 28, 19683,303,794 2/1967 Hagemann 103/162 [45] Patented Oct. 27, 1970 3,396,6708/1968 Baits 103/162 2,731,569 l/1956 Cardillo et a1. 103/38 PrimaryExaminer-William L. Freeh 1 giggig SYSTEM Aztorney- Dominik, Knechteland Godula [52] U.S.Cl 91/506,

-- 91/475 ABSTRACT: An axial piston pump drive hanger system, [51]lnt.Cl F04o 15/00, ump, and drive motor having a plurality of axiallyaligned F0413 1/02, F0413 reciprocating pistons, the piston rodconnections to a wear [50] Field of Search 103/162, plate and driveflange being characterized by a central bore in 1621A), 162(3)2 230/178;92/571 91/199 the piston rod and connecting pads all of which permithydro- References Cited static fluid from the head of the piston to betransmitted through the connecting rod through a hydrostatic button con-UNITED STATES PATENTS tact with the wear plate. A hydraulic strokecontrol system 1,736,754 11/1929 Thoma et a1 103/162 spring biased tozero displacement is also disclosed, and a 2,241,701 5/ 1941 Doe 103/162 thrust isolated drive shaft connects to a planetary transmission2,945,449 7/1960 Le Febur et al 103/162X with a rotating outer housingsuitable for connection to a 3,056,358 10/1962 Pedersen et al 1. [03/162vehicular wheel.

Patented Oct. 27, 1970 3,535,984

INVENTOR. Arthur 1-. Anderson BY fi 4 52 1 1 44 Affys.

Patehtecl Oct. 27, 1970 3,535,984

sheeti e FIG. 4

25 28 FIG 5 45 44 4 46 4/ 29 3/ 26 36) 42 25 v Z a ;:::::I-

43 32 I NVENTOR. 34 3 39 38 rthur Anderson- AHy Patented Oct. 27, 1970 I3,535,984

INVENTOR. Arthur F. Anderson TQ an el/cl Affys.

Patented Oct. 27, 1970 FIG. 11

Sheet I INVENTOR. Arthur I? Anderson v BY gw/ wl d Affys.

Patented Oct. 27, 1970 3,535,984

Sheet 5 of6 FIG. 12

INVENTOR. Arthur F Anderson Affys.

Patented Oct. 27, 1970 3,535,984

Shee t 6 0f 6 L i-L J Aawimss r FIG. 13 Z W m I RETURN FROM UNITIZEDPUMPS 55%;? R

UNITIZED WHEEL DRIVE INVENTOR. Arthur E Anderson Affys.

AXIAL PISTON PUMP-MOTOR SYSTEM The present invention relates to an axialpiston pump motor system. More particularly the invention includes thehanger hydrostatic bearing system, and the structure rendering the samestable at high speeds, and efficient and durable at high loads.

Axial piston pump drives which may be used as either a pump or a driveare known in the prior art, such as exemplified in the U.S. Pats. ofJanny, such as No. 924,787 issued June 15, I909. Lubrication systems forsuch units are also shown as exemplified in No. 2,455,330. In the vastmajority contemporary application of these structures complex rollerbearing structures are employed to translate the reciprocating motion ofthe pistons into the rotary action of a drive shaft. While thesestructures exhibit relatively low starting friction characteristics,they are efficient and long-lived primarily when operated withrelatively low loads. In addition, the bulk of the prior art axialpiston hydraulic units when employed as a motor are relatively large insize as well as heavy for a given horsepower output.

It is thus one of the principal objects of the present invention toprovide a drive unit of the axial piston type which is highly compact tothe extent that substantial horsepower capacities can be buried withinthe hub of a vehicle wheel. A related object of the invention looks tothe provision of a structure which, with minor parts and portingmodification, can be converted into a pump for driving the in-wheelhydraulic motor.

Still another object of the invention is to provide an axial piston pumpdrive in which a substantial parts reduction over the prior art isachieved, without sacrificing the mechanical efficiency achieved at lowas well as high loads and corresponding horsepower outputs.

Still another and more detailed object of the invention is to maximizethe horsepower output for a given size and weight of axial piston drivehydraulic motors.

A more detailed object of the invention is to provide an axial pistonpump drive hanger system which achieves the foregoing advantages withinfinitely variable stroke control and reversing capability. Anothermodification permits the construction of an identical unit without partschange to a fixed displacement pump and/or drive. In addition, hydraulicor manual stroke control may be employed in the drive and/or pump.

The invention stems in part from the discovery that the hydrostaticpressures within the piston may be bled through the connecting 'rod andemployed to lubricate as well as float the piston connecting rodengagement with the inclined wear plate of the drive flange. Thestructure utilized is dimensionally stable in both static and dynamicconditions thereby contributing to long life, and permitting a maximumapplication of torque and speeds which commensurately elevate thehorsepower capacities for a given unit. Furthermore, by floating theprinciple driving elements on a thin film of the hydraulic mediumworking pressures greatly in excess of those tolerable by antifrictionrolling type bearings can be achieved.

The invention, in part, is predicated upon the relationship between adrive shaft and a drive wheel, and a planetary gear assembly, as well asthe suspending of the drive shaft within the unit. By positioning therelative parts in accordance with the present invention, the pinion ofthe planetary gearing is oriented within the revolving housing in such amanner that the rotational forces are primarily radial. Thereby theaxial piston motor assembly drive may be freely supported by one set ofroller bearings and a single thrust bearing to insure uniform gearloading. The uniform gear loading results from the freefloating actionof the pinion in the planetary gear as sembly. With such gear loadingdrive efficiencies are maximized, and distortional wear tendencies areminimized.

The invention will be more fully understood as well as its objects andadvantages set forth above, when taken in conjunction with theaccompanying illustrative drawings, in which:

FIG. 1 is a perspective assembled view of a pump embodying the subjectinvention.

FIG. 2 is a perspective assembled view of a drive motor susceptible formounting inside a wheel illustrative of the present invention.

FIG. 3 is a perspective exploded view of the principal elements of themechanism employed in the pump or motor of FIGS. 1 and 2.

FIG. 4 is a transverse sectional view of a pump embodiment illustrativeof the present invention.

FIG. 5 is an enlarged partially broken sectional view of the piston andits connection to the drive flange.

FIG. 6 is a front view of the drive flange, partially sectioned toillustrate the universal joint connection between the drive flange anddrive shaft. I

FIG. 7 is a transverse broken section in enlarged scale taken throughsection lines 7-7 of FIG. 6 illustrating the universal joint connectionbetween the drive flange and drive shaft shown in the neutral position.

FIG. 8 is a view similar to FIG. 7 illustrating the tilted maximumdisplacement relationship between the drive flange and the drive shaft.

FIG. 9 is an enlarged end view of the hydrostatic button which connectsthe lower end of the piston connecting rod to the wear plate within thehanger block.

FIG. 10 is a side view of the hydrostatic button shown in FIG. 9.

FIG. II is a transverse sectional view ofa variable displacement motor,transmission, and drive wheel illustrative of the invention.

FIG. 12 is a transverse sectional view of a fixed displacement motor andtransmission for wheel mounting taken along the same section as FIG. 11.

FIG. 13 is a schematic circuit diagram showing the relationship betweenthe drive pump, unitized wheel drives, filter center. and charge andauxiliary pumps.

FIG. 14 is an enlarged sectional view of the stroke control piston andorienting spring at its fully retracted position.

FIG. 15 is an enlarged view corresponding to FIG. 14 showing theconnecting rod disengaged from the hanger and centered forrepositioning.

As will be noted in FIGS. 1 and 2 of the drawings, the pump 10 as shownin FIG. I is compact and has mounted immediately thereon a charge pumpand auxiliary pump assembly 13. A pump of this character capable ofdelivering l00 horsepower weighs approximately l5 pounds, and fitswithin a 5-inch cube.

Operating pressures to 10,000 p.s.i. and speeds to l2.000 r.p.m. arepossible. As pointed out above, a charge pump as well as an auxiliarypump is conveniently mounted on the same drive shaft. The motor 11 shownin FIG. 2 is cylindrical in configuration, and lends itself ideally tothe insertion within a drive wheel of such products as straddlecarriers, special purpose loaders, and track laying equipment.Additionally, it will be appreciated that with the high shaft rpm. andgearing employed, drive units with very low speeds can be developed foruse around equipment servicing jet aircraft. Nevertheless, by varyingthe displacement of the drive unit through an infinitely adjustablecontrol and reversing mechanism, good speeds can be obtained fortransporting the equipment from one job unit to another.

Referring now to FIG. 13, it will be observed that the pump 10 andunitized wheel drive 11 are connected by conventional hydrauliccircuitry. No special hose lines are required, nor special fittings, allbeing available commercially and oftentimes employed on existingequipment. The power center 12 includes the pump 10 as well as a chargepump 15 and auxiliary pump 16. All three pumps are driven off the maindrive shaft 14, the charge pump 15 serving to deliver fluid from thefilter center 18 directly to the main pump 10, thereby minimizing thesize of the filter center and/or reservoir. The auxiliary pump 16 isprovided to drive related equipment, but may also be employed forhydraulically driving the stroke control and hydraulically reversingeither the drive pump 10, or the unitized wheel drive 11.

Turning now to FIG. 3, it will be seen that the unit contemplates threebasic assemblies. The housing is the large outer casting 20, closed atone end by the cover assembly 21. A semitrunion 22 is integral with thecover 21, and serves to support the bearing race 24 of the hangerassembly 25. Integral with the hanger assembly is the cylinder block 26which houses a plurality of reciprocating pistons in the individualcylinders 27. The drive flange 28 is operatively coupled to the maindrive shaft as is the cylinder block 26 as will be detailed hereinafter.A stationary valve plate 29 abuts the end of the cylinder block 26 whichincludes a plurality of ports which are positioned for relative open andclosed timed communication with the ports in the valve plate 29.

Turning now to FIG. 4, it will be seen that the pump has a cylindricalor boxlike housing which is closed at one end by the cover 21. A driveshaft 30 runs through the center of the pump and is radially suspendedtherein by the universal joint 70, one end protruding for the connectionto a drive motor, and the other end may optionally extend through thehousing 20 to join the auxiliary and charge pump assembly 13 such asshown in FIG. 1, but omitted from P10. 4.

Referring now to FIG. 5, it will be seen that each piston 31 is mountedfor reciprocation within the individual cylinder 27 in the cylinderblock 26. The valve plate 29 selectively communicates with thehydrostatic fluid source (not shown) to permit high pressure fluid toenter into the cylinder at the upper end of the piston 31 to drive thesame. As the piston 31 reciprocates in the cylinder 27, the connectingrod 32 also reciprocates and transmits its force onto a wear plate 33.The wear plate 33 is positioned within the drive flange 28 which iscoupled by means of a universal joint assembly to the drive shaft 30 aswill be described hereinafter. ln greater detail, it will be seen thatthe piston 31 has nesting in its piston bore 38 a socket 34 whichengages the connecting rod piston end rounded portion 35, and is securedin place by a piston connecting rod keeper 36 which snapfittinglyengages the piston keeper groove 39 defined in the piston bore 38. Oncethe piston connecting rod keeper 36 is snapped into place, theconnecting rod 32 is fixedly and yet partially universally swiveledwithin the piston 31, and cannot be removed without special tooling, andaccordingly is dimensionally both statically and dynamically securedwithin the piston 31. The dotted lines in FIG. 5 show a borelongitudinally through the entire length of the connecting rod 32, aswell as a bore through the center of the piston 31 and the center of thesocket 34. As the description of the drive flange connection proceeds,it will become apparent that an open hydraulic circuit is provided tobleed off a portion of the hydraulic fluid from the driving end of thepiston 31 until the same floods the wear plate interface with thehydrostatic button 42, and in operation provides a thin film of fluid ofapproximately .0002 inches to constantly lubricate the wear plate 33 aswell as reduce friction at the wear point. Furthermore, because the thinfilm of oil constantly lubricates the interfacial relationship betweenthe hydrostatic button 42 and the wear plate 33, as increased pressuresare applied, an increasing pressure gradient occurs across the face ofthe hydrostatic button 42, and thereby permits load transfers up to ashigh as 15,000 lbs. p.s.i. at the driving point, corresponding loadsbeing intolerable by conventional antifriction bearing structures.

It will be noted that the connecting rod flange end 40 is spherical, andrests within the flange drive socket 41. A keeper ring for the flangedrive socket 44 is provided at the inner portion of the connecting rodflange end 40, and abuts a shoulder 43 which thereby dimensionallystablizes the connecting rod flange end 40 within the drive flange 28. Aholddown ring 45 is secured to the outer portion of the drive flange 28,and a holddown ring snap ring 46 in the hanger longitudinally securesthe relationship between the drive flange 28 and the hanger 25. All ofthe elements described above are repeated nine times as will appear fromFIG. 6, and end view of the drive flange 28.

As set forth above, it will be seen that despite the reciprocation ofthe connecting rod 32, a continuous open communication is kept for thetransmission of high pressure hydraulic fluid through the piston,connecting rod, and on to the hydrostatic button to the end that thehydrostatic button rides on a film of pressure gradient fluid againstthe wear plate 33. in the position shown in FIG. 5, the piston 31 hasjust completed discharging its displacement of pressure fluid throughthe valve plate 29 into the system. The next of motion will see thepiston 31 sucking low pressure fluid into the upper end of the cylinder27 at which time the keeper for the flange drive socket 44 retains theconnecting rod flange end within the drive flange 28. During thisportion of the cycle the pressure gradient between the hydrostaticbutton 42 and the wear plate 33 is of no major significance. During thesubsequent 180, however, as the piston 31 begins pressurizing the fluidat its head, a predetermined portion of the fluid will pass through thelongitudinal bore of the connecting rod 32, and migrate through thehydrostatic button central bore to provide the lubricating film at theinterface between the hydrostatic button 42 and the wear plate 33.Reversely, of course, when the unit is operating a motor, as the piston31 moves downwardly, high pressure fluid will migrate through thecentral bore of the connecting rod 32, and similarly coat the interfacebetween the hydrostatic button 42 and the wear plate 33 with a film oflubricant approximately .0002 inches thick; subject to a pressuregradient which varies in direct proportion to the load between thehydrostatic button 42 and the wear plate 33. In all instances, the loadis transmitted by the drive flange 28 when employed as a pump to theconnecting rod 32, by the flange drive socket 41. In the one instance,the load is transferred from the drive flange to the connecting rod whenemployed as a pump, and reversely transferred from the connecting rod tothe flange drive socket and then to the drive flange when operating as amotor.

in order to vary the total displacement of the pump, the hanger assembly25 is universally swiveled to the drive shaft 30 by the universal jointassembly 70. At diametrically opposed positions on the hanger 25,sockets 59 are provided to couple with the stroke control connecting rod52 in the stroke control assembly 50. The stroke control assembly 50 1Smotivated primarily by the stroke control piston 51, the connecting rod52 for which has substantially identical partially spherically roundedends, both of which nest within substantially identical sockets 59 (seeFIGS. 14. 15). The stroke control connecting rod 52 is provided with astroke control positioning spring 54, a coil spring, which clampinglyengages a positioning spring collar 55 on the stroke control piston 51.A positioning spring shoulder 56 is provided on the connecting rod 52,thereby permitting the stroke control positioning spring 54 to bias thestroke control piston 51 and the connecting rod 52. When it isappreciated that there are two stroke control assemblies 50 atdiametrically opposed positions, and the hanger 25 is pivotally mountedperpendicular to the drive shaft 30, it will be seen that a zerodisplacement configuration is achieved when no pressures are fed intothe stroke control assembly 50 in order to move the stroke controlpiston 51. As pointed out in connection with the description of thesystem shown in FIG. 13, the auxiliary pump 16 is employed to deliverpressure fluid to the stroke control piston 51, selectively, to the endthat the angular position of the hanger 25 may be carefully controlled.In the position of zero angularity, of course, there is a zerodisplacement neutral positioning. At one angle, the pump will dischargefluid from one port, and at the opposite angle, from another port.Similarly, when employed as a motor, infinitely variable stroke may beachieved. as well as reversibility through the infinitely variable rangeby employing same stroke control assembly 50.

It will be further appreciated from FIGS. 14, 15 that the stroke controlbiasing spring 54, because it will reorient its connecting rod 52axially with the stroke piston 51 if the connecting rod 52 disengagesits hanger socket 59, realigns the same for subsequent coupledengagement between the piston 51 and the hanger socket 59. A connectingrod keeper 58 of similar design to the piston connecting rod keeper 36(see FIG. 5) is employed to secure the piston end of the stroke controlconnecting rod 52 within its socket 59. Similarly the drive flange 28 isheld against dislodgment from the hanger 25 by means of a holddown ring45, a holddown ring snap ring 46, but additionally the holddown ringlock pin 60 is provided to prevent the snap ring 46 from rotating as thedrive flange 28 rotates in the hanger 25, the latter remainingstationary relative to the rotation of the drive flange 28. Naturallythere is no rotation of the stroke control assembly with reference tothe hanger 25. Similarly, a valve plate lock pin 61 is provided tosecure the valve plate 29 against rotation, while the cylinder block 26rotates in unison with the drive flange 28.

While the hydrostatic button 42 was referred to and described as to itsfunction above, it will be observed from FIGS. 9 and that thehydrostatic button 42 includes a hydrostatic button relief area 62 whichis surrounded by the hydrostatic button balance seal pad 64. an annularring around the periphery of the hydrostatic button and which has thebasic interfacial relationship with the wear plate 33. The hydrostaticbutton orifice 65 extends through the hydrostatic button swivel 66, andtransmits the pressure fluid from the longitudinal bore within theconnecting rod 32 into the hydrostatic button relief area 62, and apressure gradient from the operating pressure ahead of the piston 31transfers across the pad 64 to the low pressure of the interior of thepump or drive unit which is in the order of IO p.s.i. or less.

As set forth above, the drive flange 28 (as shown in FIG. 6) isconnected by means of a universal joint assembly 70 to the drive shaft14. The details of this universal joint assembly 70 are shown in FIGS.6, 7, and 8, which will be hereinafter described in greater detail.First, it will be seen that a splinelike structure forms the universaljoint inner race 71 on the drive shaft 14. The inner race 71 has acylindrical groove 72, and the drive flange outer race 74 is formed by acylindrical groove 75 within the drive flange 28. It will be observed(see FIG. 6) that the universal joint elements are on offset radialspacing from the hydrostatic buttons 42, there being nine of each. Theuniversal joint drive ball 76 is mounted on a drive ball positioner 78.The drive ball positioner 78, as will be best observed in FIGS. 7 and 8,includes a drive ball positioner stem 79, which terminates in a driveball positioner cross head 80. The cross head 80 (see FIG. 6) has acylindrical outer surface which slidably engages the cylindrical groove75 in the drive flange outer race 74. The connection to the drive shaft14 is achieved by means of the drive ball stem pivot 81 which nestswithin the drive ball stem pivot hole 82 in the drive shaft 14. It willbe appreciated from the relationship between the angles shown in FIGS. 7and 8, that a mirror image of the angle relationship in FIG. 8 may alsobe achieved. The universal joint transmits the rotary torque of thedrive flange 28 which, as set forth above. is transmitted through theconnecting rod 32 to the flange drive socket 41 by means of theinteraction between the hydrostatic button 42 and the wear plate 33.

Another important advantage achieved by the universal joint assembly 70will be appreciated when the variable displacement drive of FIG. 11 isconsidered in its assembled relationship. It will be noted that theentire drive assembly includes a rotatable housing 109 and stationaryhousing 101 with a single set of radial bearings 110 to suspend the twofor relative rotation. Because the universal joint assembly 70 issuspended by the hanger with relationship to the stationary housing 101,a single thrust bearing 112 positioned along the drive shaft absorbs theaxial thrust of the drive shaft 30 isolating the same from the planetarygear assembly. Therefore the pinion 106 of the planetary gear assemblywhich is directly coupled to the drive shaft 30 transmits only radialforces. The freely supported operation of the pinion within theplanetary transmission insures uniform gear loading. The uniform gearloading, in turn, maximizes power distribution, and minimizesdistortional wear characteristics.

Because of the related torques and the necessity for constant springloaded pressure between parts, it will be seen in FIG. 4 that a driveshaft thrust assembly has been provided to constantly urge the driveshaft away from the cylinder block, or conversely, to maintain aconstant pressure on the cylinder block 26 against the valve plate 29.As will be seen, a drive shaft thrust cylinder block spring 91 ridesagainst a cylinder block keeper 92 at one end of the spring. and biasesitself at the other end at the spline spring shoulder 94 of the cylinderblock shaft spline 68.

While the description thus far has been primarily directed to a pumpconstruction such as that shown in FIG. 4, as will be observed in FIG.11, the identical structure and interchangeable parts may be provided ina variable displacement motor which is highly compact, and contains itsown transmission which gears down the high speed rotation of the driveshaft 14 to hub speeds which are compatible with conventional vehicularusage. As will be seen in FIG. 11, the variable displacement motor 100has a stationary housing 101 which is fixedly secured to the vehicle. Afixed internal gear 102 forms part of the inner housing, terminating infixed internal gear teeth 104, As will be seen, a planet cluster gear105 engages the fixed internal gear teeth 104, and transmits byrotatable connection to the pinion gear 106, its radial force to theoutput internal gear 108. The output internal gear 108 is fixed to therevolving housing 109, and journaled by means of the load bearingassembly 110 to the stationary housing 101 An oil seal 111. only onebeing required. effectively seals the interior portion of the variabledisplacement motor 100 from dirt, as well as oil leakage of the lowpressure interior fluid.

It will also be noted that a single thrust bearing assembly 112 isprovided at the connecting point between the fixed in ternal gear 102and the gear assembly which connects to the revolving housing 109. Byproviding a thrust bearing at this point, only radial torque istransmitted to the planetary gear assembly 114, the same being securedby means of planet carrier thrust bearings 115.

The fixed displacement motor disclosed in FIG 12 is substantiallyidentical with the construction of the variable displacement motor shownin FIG. 11, except that the stroke control assembly 50 is omitted, and,instead, the drive flange fixed support recess 122 in the fixed internalgear 121 supports the drive flange in fixed angular relationship therebyfixing the displacement of the drive motorv In review it will be seenthat a system and structure has been shown and described for an axialpiston pump-drive construction. Basic to all of the structures. whetherpump or drive motor, is a hydrostatic connection between the workingarea of each individual piston and its thrust button engagement with thedrive plate of the drive flange. All of the elements have been securedagainst dislodgement and dynamically lock each to the other to the endthat assembly is facilitated, and dynamic close relationships aremaintained even at speeds as high as 12,000 rpm. Where variabledisplacement is desired in either pump or drive, the same is coupled tothe common hanger, and centeringly spring loaded to the end that failureof the auxiliary pump will zero out the displacement and the motor orpump will idle in a safe condition. By varying the control systems, themotor can serve as a brake, the pump can serve as a brake, and each isselectively reversible. Furthermore, infinite ranges of displacement andthus drive speeds and torques may be provided. Finally, because of thehydrostatic nature of the bearing assembly in the drive, maximumhorsepower can be transmitted in an inexpensive structure, andsufficiently compact to serve as a motor drive within a conventionalvehicular wheel.

While the invention has been described in connection with specificembodiments and applications, no intention to restrict the invention tothe examples shown is contemplated, but to include within the inventionall of the subject matter defined by the spirit as well as the letter ofthe annexed claims.

I claim:

1. An axial piston pump having a plurality of pistons within a cylinderblock in load engagement with an inclined wear plate supported by ahanger, and hydrostatic button means in structural and hydrauliccommunication with the piston, including pad means at the base of thehydrostatic button for load engagement with the wear plate, the systembeing proportioned to bleed pressure fluid to the interface between thehydrostatic button and wear plates as a function of the pressure fluidon the piston thereby delivering a fllm of hydraulic fluid to float thehydrostatic button on the wear plate in a proportional relationship tothe piston load, the improvement comprising stroke control means,including: a pair of pistons diametrically flanking the cylinder block,conduit-cylinder means for selectively delivering pressure fluid to theaforesaid pistons for moving the same, connecting rods connectedswivelably to the hanger and piston, a shoulder on the connecting rod,and a centering spring biasing the connecting rod shoulder and pistonwhereby the hanger is returned to a neutral position in the event ofauxiliary pressure failure to the stroke control piston, the springfurther serving to insure pressure contact between the connecting rodand hanger.

2. A paraxial multipiston hydraulic pressure mechanism having a cylinderblock, a drive flange, a hanger, and means for translating thereciprocation of the piston to an angularly adjustable wear plate in thehanger, a hydrostatic lubrication and stroke control system comprising:port means in the piston head, a connecting rod having a centrallongitudinal bore, means coupling the connecting rod central bore inopen communication with the piston port, a hydrostatic button inflatface relationship with the wear plate, and port means in thehydrostatic button proportioned to bleed fluid into the base of thehydrostatic button at the interface with the wear plate, stroke controlmeans comprising a pair of pistons diametrically flanking the cylinderblock, conduit-cylinder means for selectively delivering pressure fluidto the aforesaid pistons for moving the same, connecting rods connectedswivelably to the hanger and piston, a shoulder on the connecting rod,and a centering spring biasing the connecting rod shoulder and pistonwhereby the hanger is returned to a neutral position in the event ofauxiliary pressure failure to the stroke control piston, the springfurther serving to insure pressure contact between the connecting rodand hanger 3. An axial piston assembly comprising, in combination ahanger, a drive shaft, a housing, a cover. semitrunions in the coverforming a hanger bearing support, a needle bearing support cradling thehanger in the cover semitrunions, means defining a drive shaft centralbore in the hanger; a cylinder block, a drive flange, bore means in thecylinder block and drive flange to receive the drive shaft; universalmeans connecting the drive flange to the drive shaft, means connectingthe cylinder block to the drive shaft, an inclined wear plate in thedrive flange, a plurality of pistons in the cylinder block, connectingrods on said pistons disposed in axial alignment with the drive shaft,conduit means selectively in open communication with the pistons, ahydrostatic button connecting each connecting rod to the wear plate,conduit means through the piston, connecting rods, and hydrostaticbutton, the same oriented and proportioned to deliver pressure fluid tothe interface between the hydrostatic button and the wear plate in adirect relation to the amount of load on the pistons transmitted to thedrive shaft through the aforesaid power train; a pair of pistonsdiametrically opposed in the housing, conduitcylinder means forselectively delivering pressure fluid to the aforesaid pistons formoving the same, connecting rods connected swivelably to the hanger andpiston. a shoulder on the connecting rod, and a centering spring biasingthe connecting rod shoulder and piston whereby the hanger is returned toa neutral position in the event of auxiliary pressure failure to thestroke control piston, the spring further serving to insure pressurecontact between the connecting rod and hanger.

4. In an axial piston pump drive hanger assembly having a plurality ofpistons within a cylinder block, said pistons being in load en agementwith a wear plate, said wear late bein supported y a hanger, theimprovement m stro e contro means comprising a pair of pistonsdiametrically flanking the cylinder block, conduit-cylinder means forselectively delivering pressure fluid to the aforesaid pistons formoving the same. connecting rods connected swivelably to the hanger andpiston, a positioning spring shoulder on the connecting rod. apositioning spring collar on the stroke control piston, and apositioning spring biasing the connecting rod shoulder and piston whileclampingly engaging the positioning spring collar on the stroke controlpiston and encircling the positioning spring shoulder on the connectingrod whereby the hanger is returned to a neutral position in the event ofauxiliary pressure failure to the stroke control piston, the springserving additionally to insure pressure contact between the connectingrod and hanger and alignment in the event ofdislodgement.

